1. Field of the Invention
The present invention relates generally to radio frequency (RF) isolation filters. In particular, the present invention relates to RF isolation filters used in passive components used in conjunction with RF transceivers, such as duplexers, diplexers or other filters types.
2. Discussion of the Related Art
FIG. 1 shows transceiver duplexer circuit 100 in an RF application. As shown in FIG. 1, transceiver circuit 100 receives an RF signal at transmitter input terminal 107. The RF signal is amplified by RF power amplifier 103 for transmission. In one typical application, RF power amplifier 103 provides +50 dBm signal power in the transmitter frequency band and unwanted intermodulation (IM) products or noise of −15 dBm in the receiver frequency band. The amplified RF signal is received into input terminal 108 of duplexer 101, where the signal is provided to antenna 102 for transmission. Antenna 102 serves both the transmitter and the receiver. An RF signal picked up by antenna 102 is received into duplexer 101, which provides the received signal at output terminal 109 of duplexer 101. The isolation between antenna 102 and duplexer input terminal 108 may provide, for example, an attenuation of 110 dB in the receiver frequency band. The isolation between antenna 102 and duplexer output terminal 109 may provide, for example, a −50 dBm power leakage in the transmitter frequency band and a −125 dBm power leakage in the intermodulation products (or noise) in the receiver frequency band. A typical isolation between duplexer input terminal 108 and duplexer output terminal 109 may be, for example, 100 dB in the transmission frequency band and 110 dB in the receiver frequency band, with a sensitivity of less than −115 dBm in the receiver frequency band. The received RF signal at duplexer output terminal 109 may be amplified by low noise amplifier (LNA) 104. As shown in FIG. 1, the amplified received signal is further filtered in receiver filter 105. The filtered signal is provided to a receiver at terminal 106.
Passive duplexer 101 has the disadvantages of being both costly and bulky, and requires manual tuning in order to achieve acceptable frequency selectivity and insertion loss.